1. Field of the Invention
The present invention relates to a pyrolytic boron nitride crucible and a method for producing the same, and particularly to a pyrolytic boron nitride crucible used in the Molecular Beam Epitaxy method or the Liquid Encapsulated Czochralski method for melting therein a metal or a compound and a method for producing such a crucible.
2. Related Art Statement
Pyrolytic boron nitride is high grade boron nitride of high purity and used for wide applications including the production of compound semiconductors and special alloys. Particularly, in the production of a compound semiconductor such as GaAs the excellent anticorrosive property and the high purity of the pyrolytic boron nitride are effectively utilized for the growth of a single crystal of a compound semiconductor containing little impurities and superior in electrical properties. For example, in the process for growing a GaAs single crystal, the pyrolytic boron nitride is used for a material for a crucible in the Liquid Encapsulated Czochralski method and is also used for a material for a boat in the horizontal Bridgeman technique. Moreover, the pyrolytic boron nitride is almost exclusively used for a material for a crucible in which a metal is melted in the Molecular Beam Epitaxy method, which is a method for growing epitaxitially a mixed crystalline compound semiconductor, such as Ga.sub.1-x Al.sub.x As on a wafer made of a single crystal of GaAs.
Such pyrolytic boron nitride has been produced through the so-called chemical vapor deposition method, as disclosed by U.S. Pat. No. 3,152,006, wherein a boron halide, such as boron trichloride (BCl.sub.3) and ammonia are used as gaseous starting materials to deposite boron nitride at a temperature of from 1450.degree. to 2300.degree. C. and at a pressure of not more than 50 Torr on the surface of an appropriate substrate. Then, the deposited pyrolytic boron nitride is separated or released from the substrate to obtain an article made of self-standing pyrolytic boron nitride. The thus obtained pyrolytic boron nitride has a structure wherein the C-planes of the hexagonal crystal lattices are oriented in the direction perpendicular to the growth direction of the deposited wall layer, and thus the properties thereof are exceedingly anisotropic. The pyrolytic boron nitride has a high mechanical strength in the direction parallel to the surface of the wall layer, but the mechanical strength thereof along the crystal growth direction is not so high that the formed wall layer tends to be exfoliated along the growth or deposition direction. Such a tendency of exfoliation along the deposition direction is a main cause for reducing the lifetime of a crucible made of pyrolytic boron nitride when used repeatedly. Problems involved in the conventional crucibles used in the Liquid Encapsulated Czochralski method and the Molecular Beam Epitaxy method will be described in detail hereinbelow.
A sinble crystal rod of a compound semiconductor, such as GaAs or InP, has been predominantly produced through the Liquid Encapsulated Czochralski method comprising the steps of melting the aforementioned compound or materials for the compound in a crucible made of pyrolytic boron nitride, covering the upper surface of the mass with molten B.sub.2 O.sub.3 liquid of high purity to be encapsulated, dipping a seed crystal of the compound in the molten mass of the compound, and then pulling up the seed crystal slowly from the molten mass to form a single crystal rod of the compound. After the completion of pulling up the single crystal, the crucible must be cooled to room temperature and the cooled and solidified B.sub.2 O.sub.3, which has served as the encapsulator and is now adhering on the interior periphery of the crucible, must be removed prior to repeated use. However, in the operation of removing the solidified B.sub.2 O.sub.3, portions of the pyrolytic boron nitride layer are frequently peeled off from the interior surface of the crucible together with the B.sub.2 O.sub.3. It is an extremely rare case where peeling of the pyrolytic boron nitride layer occurs uniformly over the interior wall of the crucible, and it frequently occurs that a portion of the interior wall of the crucible is peeled off and the peeled fragment has a random thickness. In the most serious case, such a local peeling extends to the exterior peripheral surface of the crucible to result in breakdown of the crucible. Even when the defect caused by peeling is not so great in depth, the interfaces between the adjacent boron nitride wall layers contact with the molten mass so that trace impurities contained in the molten mass are collected in the interfaces. Such locally collected or concentrated impurities cause disadvantages in that the impurities are released in the molten mass at a later operation cycle or a further peeling propagates from such a location. In order to exclude such disadvantages, it is required to remove the interior wall layer until a smooth periphery is formed at the depth of the deepest defect caused by peeling. However, in this operation of forming a smooth interior periphery, a deeper defect is apt to be formed by peeling. For these reasons, the wall of the crucible becomes thinner as it is repeatedly used for growing therein a semiconductor crystal, although it is not broken due to a single peeling. As a result, the lifetime of the conventional crucible has been used up after several to ten time usages thereof.
When the pyrolytic boron nitride is used as a material for a crucible used in the Molecular Beam Epitaxy method and a metal having good wettability with the pyrolytic boron nitride, such as aluminum, is melted therein, the crucible is cooled at the time of stopping the Molecular Beam Epitaxy system, whereupon a severe stress is applied on the crucible due to tremendous differences in thermal expansion coefficients between the crucible and the metal contained therein. Since the cooled and solidified metal adheres firmly to the interior wall surface of the crucible, because of good wettability of the metal with the crucible, the crucible is often broken by the stress.
In order to solve the aforementioned problems, a multi-walled crucible having a thick outer wall layer for providing integrity of the whole structure, an intermediate wall layer and an innermost wall layer, which layers have thin thicknesses and are weakly bonded to the outer layer, has been proposed, for example by U.S. Pat. Nos. 3,986,822 and 4,058,579, and commercially sold. Although such a multi-walled crucible is successfully used as a crucible for melting a metal in the Molecular Beam Epitaxy method, the metal contained therein oozes toward the outside of the interior wall layer to make further use thereof impossible once the innermost layer has been damaged, as has been clearly described in the technical information by Union Carbide Co., U.S.A. The conventional multi-walled crucibles have been produced by a cumbersome and time-consuming process including the steps of depositing one wall layer, interrupting the deposition reaction by cooling, and then re-starting the deposition reaction by raising the temperature to the deposition temperature.